Devil Bringer

The Devil Bringer is Nero's right arm, and the physical manifestation of his demonic power. The arm possesses superhuman strength, which it can also manifest through a larger, spectral arm. It is also capable of absorbing items of power, such as the Yamato or Evil Legacy, and manifesting their power.

Throughout the game Nero displays discomfort with his arm, hiding it from other people and even admitting to Berial that "it drives him crazy". In the end he comes to accept it, telling God that he had resented the change, but now sees it as a tool to be used against evil.

Powers
The most notable feature of the Devil Bringer is, of course, its unusual ability to summon a larger, spectral arm. This arm can do feats of supernatural strength, which Nero often uses to his advantage when fighting demons. Nero regularly uses it to move and throw objects much larger than him, a technique that is known as Buster. Even Dante once admits that in brute strength Nero might be stronger than him. The arm can also be used to pull objects towards Nero (Snatch) or push him toward them instead (Hell Bound). While Devil Triggered, the ghostly arm becomes a part of the spectre.

The Devil Bringer itself also seems to be quite durable, which, combined with its unnatural strength, is very useful in blocking attacks. In fact, the only item shown to be capable of damaging it is Yamato, a powerful Devil Arm which is known for being extremely sharp. Nero does not wield any weapons with his Devil Bringer, aside from Yamato, but he does channel energy into Blue Rose with it. The claws are sharp enough to leave marks, but Nero never uses them intentionally.

Although the Devil Bringer is constantly glowing, the strength of the glow appears to be linked to several factors. Those include how much focus Nero is putting into it, Nero's own stamina and emotional state, and presence of some important objects (like Dante or Yamato). It goes out completely only once, when it is stabbed by Yamato, which also had a profound effect on Nero himself.

The Devil Bringer is able to absorb certain Key Items and use their power to grant itself new abilities. These items are obtained either by finding them within special magical pillars, or by taking them from specific bosses. Those Key Items include:
 * Evil Legacy - grants Snatch and Hell Bound;
 * Rusalka Corpse - grants Auto-Search;
 * Sephirothic Fruit - grants ability to destroy Strange Trees;
 * Aegis Shield - grants Hold.

Although Anima Mercury is never actually consumed by the Devil Bringer, it grants Nero the ability to control Gyroblades using Buster.

Before Devil May Cry 4
Although it has not been explicitly explained how Nero received the Devil Bringer, there is an Order of the Sword report stating that Nero received an injury to his shoulder before the events of the game. According to Nero, "From that day forth, (his) arm changed, and a voice echoed 'Power, give me more power' ".

Devil May Cry 4
After Nero "received" his Devil Bringer, he kept it hidden in a sling and bandages until he first encounters Dante. The Devil Bringer emits a strange glow before the former shoots the Vicar, Sanctus. Nero then reveals his arm to block a thrust from Rebellion, after he was left alone in the opera house to battle Dante.

After his encounter with Berial, the weakened demon wonders if the arm meant Nero was not human, to which Nero replies "Don't ask".

After defeating Agnus for the first time, he shows great reluctance in allowing the mad scientist to even look at the demonic arm, while Nero is keeping them separated via his sword. However, Agnus surprises him with a squad of Gladius and Bianco Angelos, and they nearly kill Nero in order to subdue him. Speared through the stomach, Nero is on the verge of death when the shattered Yamato, suspended in an energy field in the center of the room, resonates with the Devil Bringer. The demon blade repairs itself and lends its power to Nero, granting him the ability to use his Devil Trigger. Miraculously healed, Nero finally accepts the power and the curse that is his Devil Bringer, and swears to use it to protect Kyrie.

However, the use of the Devil Trigger causes the Order to send Nero's foster brother Credo to apprehend Nero, which ends in the latter's defeat. Unfortunately Kyrie witnesses this and grows fearful of Nero, which allows Agnus to capture her in order to bait the hero into following him. Nero almost rescues Kyrie once but she is once again taken out of his reach by Sanctus.

Following Dante's rescue of Nero, the younger Devil Hunter uses his power to free Kyrie from The Savior and again in the final showdown with the giant statue, which he ends by crushing its head in his spectral hand.

After the Savior is finally defeated, Nero is still ashamed to show his demonic arm to Kyrie. But, Kyrie expresses her total acceptance of Nero. In one of the last scenes, Kyrie is shown taking his right hand into hers, and the Devil Bringer softly changing its glow to golden.